1. Field of the Invention
The invention relates to a method for the determination of the viscosity of a liquid, e.g. of blood, which is forwarded by a pump, e.g. a blood pump.
2. Description of the Prior Art
Pumps, in particular blood pumps, which are used for example in heart-lung machines (as a substitute for the pumping function of the heart) have an inlet and an outlet for the liquid to be forwarded, thus here the blood. Furthermore, pumps of this kind have a rotor which forwards the liquid to be forwarded, thus here the blood, from the inlet to the outlet. Especially in blood pumps, which are used in heart-lung machines, one of the parameters which must be continuously monitored is the viscosity of the blood. But the monitoring of the viscosity can also be required or desirable respectively in pumps other than blood pumps.
A blood pump in which it is possible to determine the viscosity of the blood is for example known from U.S. Pat. No. 5,725,357. In the pump described there the rotor is actively magnetically journalled in the axial direction. A part of the magnetic bearing is admittedly formed with the help of permanent magnets; the other part of the magnetic bearing is however formed with the help of regulatable electromagnets in order to be able to actively regulate the bearing, which is formed of two parts.
The viscosity determination is carried out in accordance with DE-A-196 13 388 in such a manner that a disturbance signal with a definite amplitude and a definite frequency is supplied to the regulation circuit for the magnetic bearing, e.g. with a frequency of 70 Hz. The rotor, which is located in a certain position without the disturbance signal, is axially deflected out of this position by the disturbance signal. If now the viscosity of the blood changes, the axial deflection of the rotor also changes particularly noticeably at the named disturbance frequency of 70 Hz. If then the dependence of the axial deflection of the rotor on the respective speed of rotation of the rotor at a frequency of the disturbance signal of 70 Hz is also taken into account, then the viscosity of the blood can be determined via the respective axial deflection of the rotor.
This blood pump or the described procedure respectively is disadvantageous in so far as an actively regulatable axial journalling of the pump rotor must be provided in order to be able to impress the disturbance signal via the active journalling. On the other hand it is not possible to determine the viscosity in the above described manner with pumps in which the axial journalling of the rotor is mechanical because the mechanical journalling of the rotor is rigid in the axial direction so that a deflection of the rotor in the axial direction can not take place. A determination of the viscosity of the blood is also not possible in this manner in passive axial magnetic bearings, in which the rotor is journalled in the axial direction through reluctance forces, because an active excitation of the journalling in passive bearingsxe2x80x94the property xe2x80x9cpassivexe2x80x9d says precisely thisxe2x80x94is simply not possible. Thus the described procedure is suitable only for pumps with an active axial magnetic journalling of the rotor; for pumps with rotors which are mechanically journalled in the axial direction and also for pumps with a passive axial magnetic journalling the described procedure is not suitable.
An object of the invention is therefore to propose a method by means of which it is possible to determine the viscosity of the forwarded liquid, e.g. blood, in a pump, e.g. a blood pump, with as little cost and complexity as possible, with the type of the journalling of the rotor, in particular the type of the axial journalling, should be freely selectable. In particular the method should naturally be suitable for magnetic journallings of the rotor which are typically used in blood pumps.
For the determination of the viscosity of a liquid, e.g. blood, which is forwarded by a pump, e.g. a blood pump, said pump having a rotor for the forwarding of the liquid from the inlet of the pump to the outlet, the rotor is likewise utilised for the determination of the viscosity. However no active excitation of the rotor for the axial deflection out of its operating position takes place for the determination of the viscosity of the liquid, but rather the rotor is left in its operating position and the viscosity is determined from the measurement parameters of the pump or of the rotor respectively. Thus the type of the journalling of the rotor, in particular the type of the axial journalling of the rotor, is freely selectable. Naturally the method is especially suitable also for magnetic journallings of the rotor, such as are typically used in blood pumps, in particular also for passive axial magnetic journallings of the rotor.
As measurement parameters the drive torque or the drive current respectively of the rotor and/or the speed of rotation of the rotor can be measured. These are parameters which can be easily measured outside the pump. This holds both for the drive torque of the rotor (or, more precisely: of the motor which drives the rotor), since the drive torque is approximately directly proportional to the drive current and a determination can be made via a known functional relationship for absolute exactness, and for the speed of rotation, which is approximately proportional to the voltage at the rotor (and a determination can be made via a known functional relationship for absolute exactness), so that for practical purposes the voltage at the rotor (or, more precisely: at the motor which drives the rotor) can be measured as a measure for the speed of rotation. In speed-of-rotation regulated motors, which are motors which are regulated to a constant speed of rotation, it is sufficient in principle to measure only the drive torque (the speed of rotation is approximately constant). In torque regulated motors, which are motors which are regulated to a constant drive torque or to a constant drive current respectively, it is sufficient in principle to measure only the speed of rotation (the drive torque or the drive current respectively is approximately constant). Naturally it is always possible and naturally the exact procedure to measure both the speed of rotation as well as the drive torque or the drive current respectively. From the values for the drive torque or the drive current respectively and/or the speed of rotation of the rotor the viscosity of the liquid is then determined.
In this a plurality of measurements of the drive torque or the drive current respectively and/or the speed of rotation of the rotor can be carried for the determination of the viscosity so that a plurality of (individual) values for the viscosity are determined. From this plurality of values for the viscosity, an average viscosity is then determined which then represents the value for the viscosity. Through this the precision of the value for the viscosity can be increased. Further method variants differ in that they are carried out either with the outlet of the pump closed or with the outlet of the pump open, thus quasi xe2x80x9con linexe2x80x9d.
In a first series of advantageous method variants the pump outlet is closed prior to the determination of the viscosity with the help of the measurement parameters. During a heart operation the outlet must be clamped off anyway for reasons of the operation so that the time in which the outlet is closed can be used for the determination of the viscosity.
In this the measurements of the drive torque or the drive current respectively and/or the speed of rotation of the rotor can be effected through a trigger signal which, after the closure of the pump outlet, is transmitted further to a control system which then initiates the measurements of the drive torque or the drive current respectively and/or the speed of rotation of the rotor.
A trigger signal of this kind can for example be produced by a closure apparatus for the closing of the pump outlet, e.g. a valve, and conducted to the control system as soon as the pump outlet is closed.
Another possibility consists in that the trigger signal is produced by a through-flow measurement apparatus which is arranged at the pump outlet and is conducted to the control system as soon as no more through-flow is determined at the pump outlet. If no more through-flow is determined at the pump outlet, this means that the pump outlet is closed and the measurements of the drive torque or the drive current respectively and/or the speed of rotation of the rotor for the determination of the viscosity can be initiated.
Yet another possibility consists in that the trigger signal is produced by a pressure measurement apparatus which is arranged at the pump outlet and is conducted to the control system as soon as a pressure is determined at the pump outlet which exceeds a threshold value at a given speed of rotation of the rotor. The exceeding of this threshold pressure also means that the pump outlet is closed and that the measurements of the drive torque or the drive current respectively and/or the speed of rotation of the rotor for the determination of the viscosity can be initiated.
Furthermore, in a pump with regulation of the speed of rotation an abrupt drop in the drive torque arising after the closing of the pump outlet can be detected and the trigger signal thereupon conducted to the control system. In a pump with regulation of the speed of rotation namely the drive torque or the drive current respectively drops abruptly after the closure of the pump outlet because no more blood can be forwarded and the blood which is located in the pump very rapidly has the same speed as the pump rotor so that only a low drive torque or a low drive current respectively is still required for maintaining this speed.
Conversely, in a pump with drive torque regulation or drive current regulation respectively an abrupt rise in the speed of rotation which takes place after the closure of the pump outlet can be detected and the trigger signal thereupon conducted to the control system. In a pump with drive torque regulation or drive current regulation respectively namely the speed of rotation rises very rapidly after the closure of the pump outlet because no more blood can be forwarded and the blood which is located in the pump very rapidly has the same speed of rotation as the rotor so that only a low drive torque or a low drive current is still required to maintain this speed. Nevertheless the same drive torque or the same drive current respectively is still available as when the pump outlet is open, which has an acceleration of the rotor (speed of rotation increase) and of the liquid as a result.
In a second series of advantageous method variants the measurements of the drive torque or drive current respectively and/or speed of rotation of the rotor are carried out with the pump inlet open and the pump outlet open, thus quasi xe2x80x9con linexe2x80x9d during the operation. This method variant can be carried out at any desired time (e.g. at any desired time during a heart operation) except at just those times at which the pump outlet is closed.
In this method variant the determination of the viscosity can take place in such a manner that the speed of rotation of the rotor is modulated about a nominal speed of rotation with a modulation amplitude and a modulation frequency, with practically no or only a slight change of the through-flow being produced at the outlet of the pump by the modulation. As a result of the inertia of the liquid, substantially no change in the through-flow takes place; however, a fluctuation in the drive torque or in the drive current respectively is produced by the modulation. The viscosity is then determined from the modulation amplitude of the speed of rotation and from the amplitude of the fluctuation of the drive torque or the drive current respectively resulting therefrom.
The determination of the viscosity can also take place in such a manner that the drive torque or the drive current respectively is modulated about a nominal drive torque or a nominal drive current respectively with a modulation amplitude and a modulation frequency, with practically no or only a slight change of the through-flow being produced at the outlet of the pump by the modulation, with however a fluctuation in the speed of rotation being produced.
Here as well substantially no change in the through-flow takes place as a result of the inertia of the liquid. The viscosity is then determined from the modulation amplitude of the drive torque or of the drive current respectively and the amplitude of the fluctuation of the speed of rotation resulting therefrom.
Since all of the above described method variants are especially also suitable for blood pumps, a pump with a magnetic journalling of the rotor is advantageously used, such is typically the case in blood pumps (for a number of reasons, e.g. contamination).